Integrated cavity output spectroscopy (ICOS) is used to measure small optical absorptions for the quantification and speciation of trace constituents. ICOS uses two or more high reflectivity mirrors to trap light and increase the effective path length. In ICOS, the light passes through the front mirror to enter the cavity and through (usually) another mirror, such as the back mirror, onto the detector. This double transmission under non-resonant conditions results in a power transmission on the order of T/2, where T is the transmission fraction of the mirrors.
The low power transmission limits instrument sensitivity and requires high power lasers and/or high gain amplifier detectors—all of which increase the cost and decrease the utility of ICOS instruments. There are several optical regions where these problems are acute. Of note, lasers operating between 3 and 4 um produce limited power, making it difficult to accurately and precisely measure the many important hydrocarbon absorptions in the region. Also, high reflectivity mirrors made to operate at wavelengths longer than about 8 um require materials in the Bragg stack that have high optical losses—the many cavity reflections that are a requirement of cavity-enhanced absorption spectroscopy (CEAS) rob the system of light, resulting in very small powers incident on the detector.
The addition of an exit hole to a standard multi-pass cell is described by Herriott et al. in “Folded Optical Delay Lines”, Applied Optics 4(8), 883-889 (August 1965). The addition of a hole for the introduction of light has also been previously proposed. In an article by Dasgupta et al., “Cavity-Enhanced Absorption Measurements across Broad Absorbance and Reflectivity Ranges”, Analytical Chemistry 86, 3727-3734 (2014), the merits of such an injection hole are detailed. These previously proposed embodiments succeed in increasing the transmitted power, but sacrifice path length as a result, yielding mere tens of reflections instead of the thousands common to standard CEAS. This is a result of limitations on spherical-spherical and planar-planar cavities that was recognized almost immediately after the introduction of multi-pass cells. In each of these cases, the stability conditions return the injected beam to the injection hole after a few passes where the power is passed out.
As a result, complicated astigmatic cells were invented that used either nearly spherical mirrors or clocked cylindrical mirrors, as in McManus et al., “Astigmatic mirror multipass absorption cells for long-path-length spectroscopy”, Applied Optics 34(18), 3336-3348 (20 Jun. 1995); Joel A. Silver, “Simple dense-pattern optical multipass cells”, Applied Optics 44(31), 6545-6556 (1 Nov. 2005). These cavities are currently used by some companies (e.g., Aerodyne, Inc) for trace gas detection.
Herriott cells have the requirement that light must be passed in through the entrance hole, reflected through the cavity and then pass out of the hole without variable loss. This limits the cavity configurations to those that have a reentrant condition, i.e., passing in and out of the same hole and makes them hard to align and operate under field conditions (vibrations, changing temperature and pressure, etc.).